1. Field of the Invention
The present invention relates to a method and a device for controlling the recording power of a semiconductor laser emitted to record marks onto an optical disc according to a data signal.
2. Description of Related Art
FIG. 8 shows the configuration of a semiconductor laser control device according to the prior art.
Light emitted by the semiconductor laser 1 is sensed by a photodetector 2 for monitoring the emission level, converted to a photoelectric current, and then converted to a voltage by a current-voltage convertor 3.
The sample-hold circuit 5 is set to a sample state by a command from write gate 8 during playback. Playback power current controller 6 compares the read power setting 7 with the detected output in the sample state and outputs the result in a feedback control loop adjusting light output from the semiconductor laser 1 to the read power setting 7.
During recording, the write gate 8 sets the sample-hold circuit 5 to the hold state, and the peak switch 10 switches between a peak value and the held read power setting to current drive the semiconductor laser 1.
The pulse beam emitted from the semiconductor laser 1 is sensed by the photodetector 2, voltage-current converted, and the envelope of the output wave peaks is detected by peak detector 4. Peak current controller 11 compares the output from peak detector 4 with peak power setting 12 and outputs to the peak switch 10 in a feedback control loop adjusting the peak value of the pulse beam from the semiconductor laser 1 to the peak power setting.
The signals from major parts of the prior art semiconductor laser control circuit are described next with reference to FIGS. 9(a)-9(d).
FIG. 9(a) shows the output from write gate 8, which goes from a playback level to a record level at time 13. FIG. 9(b) shows the output from peak detector 4, that is, detecting the peak envelope of the pulse wave sensed in the record state. FIG. 9(c) shows the waveform of the light pulses feedback-controlled by the peak current controller 11. FIG. 9(d) shows the envelope of the playback wave reproduced after this feedback controlled light pulse train is recorded to a track of the optical disc.
The optical disc referred to here is an optical disc such as DVD-R or DVD-RW media, that is, media that can preferably be recorded in multiple sessions with the signals written continuously to the recorded tracks in the same way that signals are recorded to DVD-ROM media. More specifically, the start of recording to an incremental data recording area and the end of the previously recorded data recording area are preferably recorded so that the envelope of the playback signal is continuous. The present invention is therefore applicable to optical disc media such as DVD-R and DVD-RW, and is not applicable to DVD-RAM media.
A problem with the configuration described above is that due to feedback control response, there is a response time delay as shown at bracket 14 in FIG. 9(c) until the peak of the emitted waveform rises to the peak power setting. When the optical disc is recorded with an emission wave as shown at bracket 14 in FIG. 9(c), the amplitude of the reproduced playback wave is degraded according to the response of the emitted waveform as shown in FIG. 9 (d).
A problem with the configuration of a prior art semiconductor laser control device as described above, therefore, is that the amplitude of the playback signal at the beginning of a data recording area appended to the disc deteriorates, and continuity of the playback signal amplitude is thus degraded when incrementally writing to DVD-R or DVD-RW optical disc media.
The present invention is directed to a solution for this problem, and an object of the invention is to provide a method and device for controlling a semiconductor laser in order to prevent playback signal deterioration when incrementally writing to DVD-R or DVD-RW optical disc media by quickly controlling and setting recording pulse power to a desirable recording power level.
To achieve this object, a method according to the present invention for controlling the optical power of light pulses obtained by modulating a semiconductor laser to at least two optical power levels according to a data signal for recording a mark area to an optical disc according to the data signal modulates light pulses with a test signal in a test emission area provided immediately before the data signal recording area for recording a data signal to an optical disc as light pulses modulated according to the data signal, obtains the difference between a reference level equivalent to a target pulse power level and a photodetection signal obtained by sensing and converting the light pulses to an electrical photodetection signal, and applies feed-forward control so that the difference converges to a specified value and current flowing to the semiconductor laser is controlled to a value equivalent to a target power level. Next, it senses the light pulses modulated according to the data signal in the data signal recording area, converts the sensed light pulses to an electrical photodetection signal, obtains a difference between a reference level equivalent to a target pulse power level and sample values obtained by sequentially sampling the photodetection signal at a specific sampling period, and applies feedback control so that the difference converges to a specified value and current flowing to the semiconductor laser is controlled to a value equivalent to a target power level.
The test emission area and data signal recording area are preferably provided in the area traced within one revolution of the optical disc by the optical head from the time at which the optical disc recording operation starts.
Further preferably, the test emissions in the test emission area include pulse emissions and continuous emissions at a constant level.
Yet further preferably, the test emissions in the test emission area include at least two of the following: pulse emissions modulated between the peak and bottom levels of recording pulses emitted at recording marks in the data signal recording area, constant emission at the pulse bottom level, and constant emission at the space level emitted at recording symbol spaces.
Yet further preferably, recording pulses in the recording mark area of the data signal recording area include a start pulse and plural multipulse trains. Feedback control controlling current flowing to the semiconductor laser to a value equivalent to a target power level is achieved by sequentially sampling at a specific sampling interval and step-by-step comparing with a reference level equivalent to a light pulse target power level only the average of the plural multipulse trains, or the average and bottom level of the multipulse trains in the photodetection signal obtained by sensing and converting to an electrical signal the recording pulses in the recording mark period, and the space level of the photodetection signal obtained by sensing and converting to an electrical signal the recording pulses in the recording symbol space part.
Yet further preferably, the data signal for recording to the optical disc is temporarily stored to buffer memory in the test emission area from the timing at which the optical disc recording operation starts to the beginning of the data signal recording area.
A further method according to the present invention for controlling the optical power of light pulses obtained by modulating a semiconductor laser to at least two optical power levels according to a data signal for recording a mark area to an optical disc according to the data signal modulates light pulses with a first test signal in a first test emission area provided immediately before the data signal recording area for recording a data signal to an optical disc as light pulses modulated according to the data signal, obtains a difference between a reference level equivalent to a target pulse power level and a photodetection signal obtained by sensing and converting these light pulses to an electrical photodetection signal, and applies feed-forward control so that the difference converges to a specified value and current flowing to the semiconductor laser is controlled to a value equivalent to a target power level. Next, it modulates light pulses with a second test signal in a second test emission area immediately following the first test emission area, senses and converts these light pulses to an electrical photodetection signal, obtains a difference between a reference level equivalent to a target pulse power level and sample values obtained by sequentially sampling the photodetection signal at a specific sampling period, and applies feedback control so that the difference converges to a specific value and current flowing to the semiconductor laser is controlled to a value equivalent to a target power level. Next, it senses the light pulses modulated according to the data signal in the data signal recording area, converts the sensed light pulses to an electrical photodetection signal, obtains a difference between a reference level equivalent to a target pulse power level and sample values obtained by sequentially sampling the photodetection signal at a specific sampling period, and applies feedback control so that the difference converges to a specified value and current flowing to the semiconductor laser is controlled to a value equivalent to a target power level.
Preferably, the first and second test emission areas are provided in the area traced within one revolution of the optical disc by the optical head from the time at which the optical disc recording operation starts, and the data signal recording area is provided in the area traced in a second revolution.
Yet further preferably, the test emissions in the first and second test emission areas include pulse emissions or continuous emissions at a constant level.
Yet further preferably, the first test emissions in the first test emission area include pulse emissions modulated between the peak and bottom levels of recording pulses emitted at recording marks in the data signal recording area, constant emissions at the pulse bottom level, and constant emissions at the space level emitted at recording symbol spaces.
Yet further preferably, the second test signal consists of recording pulse emissions of random two-value data converted to a specific modulation sign at plural levels required for recording mark formation.
Yet further preferably, recording pulses in the recording mark area of the data signal recording area include a start pulse and plural multipulse trains. Feedback control controls current flowing to the semiconductor laser to a value equivalent to a target power level by sequentially sampling and step-by-step comparing with a reference level equivalent to a light pulse target power level the average and bottom level of the plural multipulse trains of the photodetection signal obtained by sensing and converting to an electrical signal the recording pulses in the recording mark period, and the space level of the photodetection signal obtained by sensing and converting to an electrical signal the recording pulses in the recording symbol space part.
Yet further preferably, the data signal for recording to the optical disc is temporarily stored to buffer memory in the first and second test emission areas from the timing at which the optical disc recording operation starts to the beginning of the data signal recording area.
A further method according to the present invention for controlling the optical power of light pulses obtained by modulating a semiconductor laser to at least two optical power levels according to a data signal for recording a mark area to an optical disc according to the data signal emits a semiconductor laser according to a third test signal in a third test emissions area provided before the data signal recording area for recording a data signal to an optical disc as light pulses modulated according to the data signal, senses and converts the test signal to an electrical photodetection signal, sequentially samples the photodetection signal at specific sampling intervals and obtains a difference between the samples and a reference level equivalent to a target pulse power level, and applies feedback control converging the difference to a specific value and controlling current flowing to the semiconductor laser to a value equivalent to a target power level. Next, it modulates light pulses with a fourth test signal in a fourth test emission area provided after the third test emission area, senses and converts the light pulses to an electrical photodetection signal, obtains a difference between the photodetection signal and a reference level equivalent to a target light pulse power level, and applies feed-forward control converging the difference to a specific value and controlling current flowing to the semiconductor laser to a value equivalent to a target power level. Next, it senses and converts light pulses modulated by the data signal to an electrical photodetection signal in the data signal recording area, sequentially samples the photodetection signal at specific sampling intervals, obtains a difference between the samples and a reference level equivalent to a target light pulse power level, and applies feedback control converging the difference to a specific value and controlling current flowing to the semiconductor laser to a value equivalent to a target power level.
Preferably, the third test emission area is provided in the area traced within one revolution of the optical disc by the optical head from the time at which the optical disc recording operation starts, and the fourth test emission area and data signal recording area are provided in the area traced in a second revolution.
Yet further preferably, test emissions in the third test emission area are continuous emissions at a constant level.
Yet further preferably, test emissions in the third test emission area are continuous emissions at the bottom level of recording pulses emitted in the recording mark area of the data signal recording area or continuous emissions at the space level emitted in the recording symbol space part of the data signal recording area.
Yet further preferably, test emissions in the fourth test emission area are pulse emissions modulated to a level between the peak and bottom levels of recording pulses emitted in the recording mark area of the data signal recording area.
Yet further preferably, recording pulses in the recording mark area of the data signal recording area include a start pulse and plural multipulse trains. Feedback control controls current flowing to the semiconductor laser to a value equivalent to a target power level by sequentially sampling and step-by-step comparing with a reference level equivalent to a light pulse target power level the average of the plural multipulse trains of the photodetection signal obtained by sensing and converting to an electrical signal the recording pulses in the recording mark period, and the space level of the photodetection signal obtained by sensing and converting to an electrical signal the recording pulses in the recording symbol space part.
Yet further preferably, the data signal for recording to the optical disc is temporarily stored to buffer memory in the third and fourth test emission areas from the timing at which the optical disc recording operation starts to the beginning of the data signal recording area.
A control apparatus according to the present invention for controlling the optical power of light pulses obtained by modulating a semiconductor laser to at least two optical power levels according to a data signal for recording a mark area to an optical disc according to the data signal has means for modulating light pulses with a test signal in a test emission area provided immediately before the data signal recording area for recording a data signal to an optical disc as light pulses modulated according to the data signal, and obtaining a difference between a reference level equivalent to a target pulse power level and a photodetection signal obtained by sensing and converting the light pulses to an electrical photodetection signal; a feed-forward control means for controlling current flowing to the semiconductor laser to converge to a value equivalent to a target power level; means for sensing the light pulses modulated according to the data signal in the data signal recording area, converting the sensed light pulses to an electrical photodetection signal, and obtaining a difference between a reference level equivalent to a target pulse power level and sample values obtained by sequentially sampling the photodetection signal at a specific sampling period; and a feedback control means for controlling current flowing to the semiconductor laser to a value equivalent to a target power level.
A further control apparatus according to the present invention for controlling the optical power of light pulses obtained by modulating a semiconductor laser to at least two optical power levels according to a data signal for recording a mark area to an optical disc according to the data signal has means for modulating light pulses with a first test signal in a first test emission area provided immediately before the data signal recording area for recording a data signal to an optical disc as light pulses modulated according to the data signal, and obtaining a difference between a reference level equivalent to a target pulse power level and a photodetection signal obtained by sensing and converting these light pulses to an electrical photodetection signal; a feed-forward control means for controlling current flowing to the semiconductor laser to a value equivalent to a target power level; means for modulating light pulses with a second test signal in a second test emission area immediately following the first test emission area, sensing and converting these light pulses to an electrical photodetection signal, and obtaining a difference between a reference level equivalent to a target pulse power level and sample values obtained by sequentially sampling the photodetection signal at a specific sampling period; a feedback control means for controlling current flowing to the semiconductor laser to a value equivalent to a target power level; means for sensing the light pulses modulated according to the data signal in the data signal recording area, converting the sensed light pulses to an electrical photodetection signal, and obtaining a difference between a reference level equivalent to a target pulse power level and sample values obtained by sequentially sampling the photodetection signal at a specific sampling period; and a feedback control means for controlling current flowing to the semiconductor laser to a value equivalent to a target power level.
A yet further control apparatus according to the present invention for controlling the optical power of light pulses obtained by modulating a semiconductor laser to at least two optical power levels according to a data signal for recording a mark area to an optical disc according to the data signal, comprising: means for emitting a semiconductor laser according to a third test signal in a third test emissions area provided before the data signal recording area for recording a data signal to an optical disc as light pulses modulated according to the data signal, sensing and converting the test signal to an electrical photodetection signal, sequentially sampling the photodetection signal at specific sampling intervals and obtaining a difference between the samples and a reference level equivalent to a target pulse power level; feedback control means for controlling current flowing to the semiconductor laser to a value equivalent to a target power level; means for modulating light pulses with a fourth test signal in a fourth test emission area provided after the third test emission area, sensing and converting the light pulses to an electrical photodetection signal, and obtaining a difference between the photodetection signal and a reference level equivalent to a target light pulse power level; a feed-forward control means for controlling current flowing to the semiconductor laser to a value equivalent to a target power level; means for sensing and converting light pulses modulated by the data signal to an electrical photodetection signal in the data signal recording area, sequentially sampling the photodetection signal at specific sampling intervals, and obtaining a difference between the samples and a reference level equivalent to a target light pulse power level; and a feedback control means for controlling current flowing to the semiconductor laser to a value equivalent to a target power level.